Field of the Invention
The present invention relates to a focus detection apparatus of an imaging apparatus, and particularly to a positional relation between a photoelectric conversion element for detecting information about a focusing state and optical images formed on the photoelectric conversion element.
Description of the Related Art
With the increase in the use of imaging apparatuses represented by digital cameras and video cameras, demands for higher definition and downsizing of imaging apparatuses are steadily increasing. In particular, an increase in field of view and downsizing are demanded for a focus detection apparatus for detecting a focusing state of an imaging lens of an imaging apparatus. A focus detection apparatus employing the Through-The-Lens (TTL) phase difference system forms a plurality of optical images with a plurality of light fluxes that passed through a plurality of division regions of an exit pupil of an imaging lens, and detects a focusing state of the imaging lens based on a relative positional relation between the plurality of optical images.
In the above-described focus detection apparatus, a photoelectric conversion element for detecting information about the focusing state detects light in a wide range of a shooting region. This enables focus detection over a wide range. The size of an optical image on the photoelectric conversion element is determined by the size of a viewing field mask for limiting a light flux entering a focus detection optical system, and the magnification of a secondary imaging lens for forming an optical image on the photoelectric conversion element. In a focus detection optical system having a large viewing field mask and a small secondary image forming magnification, the photoelectric conversion element can detect light in a wide range of the shooting region. However, with an increase in size of an optical element for guiding a light flux in a wide range to the photoelectric conversion element in the focus detection optical system or in size of the housing of the focus detection optical system, an increase in size of the focus detection apparatus may result. Further, a decrease in magnification of the secondary imaging lens degrades the sensitivity of the focus detection apparatus to fluctuation of a relative positional relation between subject images on the photoelectric conversion element plane with respect to defocusing. Thus, the focus detection accuracy decreases. Japanese Patent Application Laid-Open No. 2006-322970 discusses a method for shortening the distance between a pair of optical images formed on a photoelectric conversion element is used for downsizing of a focus detection apparatus. However, if adjacent optical images are overlapped with each other, the adjacent optical images cause noise resulting in degraded focus detection accuracy.
As described above, three elements “wide field of view”, “high accuracy”, and “small size” are in a trade-off relation on a principle basis. A focus detection apparatus having a suitable balance between the three elements is demanded.
A focus detection apparatus discussed in Japanese Patent Application Laid-Open No. 2006-322970 achieves both an increase in width of field of view and downsizing. More specifically, the shape of the viewing field mask is designed in such a manner that an overlapped region is not formed between optical images on a photoelectric conversion element, and further an imaging region is not provided in a region where optical images are close to each other. However, shortening the distance between optical images degrades the sensitivity of the focus detection apparatus to fluctuation of a relative positional relation between subject images on the photoelectric conversion element plane with respect to defocusing. Thus, detection accuracy degradation may result.
The present invention is directed to providing a focus detection apparatus capable of performing focus detection in a wide field of view while avoiding detection accuracy degradation and size increase.